The cetane number of a fuel composition is a measure of its ease of ignition and combustion. With a lower cetane number fuel a compression ignition (diesel) engine tends to be more difficult to start and may run more noisily when cold; conversely a fuel of higher cetane number tends to impart easier cold starting, to lower engine noise, to alleviate white smoke (“cold smoke”) caused by incomplete combustion after.
There is a general preference, therefore, for a diesel fuel composition to have a high cetane number, a preference which has become stronger as emissions legislation grows increasingly stringent, and as such automotive diesel specifications generally stipulate a minimum cetane number. To this end, many diesel fuel compositions contain ignition improvers, also known as cetane boost additives or cetane (number) improvers/enhancers, to ensure compliance with such specifications and generally to improve the combustion characteristics of the fuel.
Organic nitrates have been known for some time as ignition accelerants in fuels, and some are also known to increase the cetane number of diesel fuels. Perhaps the most commonly used diesel fuel ignition improver is 2-ethylhexyl nitrate (2-EHN), which operates by shortening the ignition delay of a fuel to which it is added.
However, 2-EHN is also a radical initiator, and can potentially have an adverse effect on the thermal stability of a fuel. Poor thermal stability in turn results in an increase in the products of instability reactions, such as gums, lacquers and other insoluble species. These products can block engine filters and foul fuel injectors and valves, and consequently can result in loss of engine efficiency or emissions control.
The organic nitrates described in the prior art as combustion improvers and/or cetane number improvers have a series of disadvantages, especially lack of thermal stability, excessively high volatility and insufficient efficacy. However, it may be expected that by decreasing the volatility of a cetane enhancer, e.g. by using a molecule of higher molecular weight, its efficacy as a combustion improver and/or cetane number improver may then decline.
There are also health and safety concerns regarding the use of 2-EHN, which is a strong oxidising agent and is also readily combustible in its pure form. It can also be difficult to store in concentrated form as it tends to decompose, and so is prone to forming potentially explosive mixtures. Furthermore, it has been noted that 2-EHN functions most effectively under mild engine conditions.
These disadvantages, taken together with the often significant cost of incorporating 2-EHN as an additive into a fuel composition, mean that it would be generally desirable to reduce or eliminate the need for 2-EHN and other known cetane number improvers in diesel fuel compositions, whilst at the same time maintaining acceptable combustion properties.
It is therefore an object of the invention to provide cetane enhancers which are effective as combustion improvers or cetane number improvers.